As evidenced below from the descriptions of the several cited patents, there are many different approaches to manufacturing printed circuit boards and cards (hereinafter also simply referred to as PCBs), chip carriers and the like substrates. This is true with respect to substrates used in high speed and other final structures. With operational requirements increasing for complex electronic components such as semiconductor chips which mount on circuitized substrates of the types cited above, so too must the host substrate be capable of handling these increased requirements. One particular increased requirement has been the need for higher frequency (high speed) connections between two or more such mounted components, which connections, as stated, occur through the underlying host substrate. By the term “high speed” as used herein is understood to mean signals within a frequency range of from about 3.0 to about 10.0 gigabits per second (GPS) and even higher.
Such high speed connections are subjected to various detrimental effects, e.g., signal deterioration (also referred to as signal attenuation), caused by the inherent characteristics of such known substrate circuitry wiring. In the particular case of signal deterioration, this effect is expressed in terms of either the “rise time” or the “fall time” of the signal's response to a step change. The deterioration of the signal can be quantified with the formula (Z0*C)/2, where Z0 is the transmission line characteristic impedance, and C is the amount of the connecting “via” capacitance (the “via” being a known plated hole within the substrate to couple different conductive layers). In a signal line (also referred to in the industry as a wire or trace) having a typical 50 ohm transmission line impedance, a plated thru-hole “via” having a capacitance of 4 pico-farads (pf) would represent a 100 pico-second (ps) rise-time (or fall time) degradation. This compares to a 12.5 ps degradation with a 0.5 pf buried “via” of the various embodiments taught in the patent application cited above. This difference is significant in systems which operate at 800 MHz or faster (becoming the “norm” in today's technical world), where there are associated signal transition rates of 200 ps or faster.
One factor that can contribute to signal attenuation is surface roughness of the conductive layer through which the signals pass. PCB manufacturers who laminate several dielectric and conductive layers to form the final board structure desire some level of roughness to promote adhesion between the two materials. Unfortunately, such roughness may also adversely affect signal passage if too excessive. As understood from the teachings herein, the instant invention is able to provide conductive layers with optimal roughness for sound adhesion to corresponding dielectric layers during bonding of such layers but also layers that are smooth enough that the surface irregularities of such layers do not significantly impede signal passage.
It is to be understood that the teachings of the present invention are not limited to the manufacture of high speed substrates such as PCBs and the like, but are also applicable to the manufacture of substrates used for other purposes than high speed signal connections. Generally speaking, the teachings herein are applicable to any such substrates in which one or more conductive layers such as copper are bonded (e.g., laminated) to an adjacent dielectric layer and the resulting composite then used as the substrate, typically when combined with other dielectric and conductive layers to form a much thicker, built-up structure. The invention is able to provide a final structure in which signal attenuation is reduced while still assuring effective conductive layer and dielectric layer adhesion.
With respect to the circuit (wiring) patterns being formed on substrates of many types of PCBs, including high speed boards as well as others, line widths may now be as small as ten-odd microns. Accordingly, the conductive layers (some also referred to as metal “foils” in the art) are becoming much thinner than those which produced wider lines in previous substrates. By way of example, when the designated thickness of metal foil for use in the formation of the conventional wiring pattern of about 100 micron line width has ranged from about 15 to 35 microns, the thickness of metal foil utilized in the formation of ten-odd micron wiring patterns must be reduced correspondingly. To accomplish this, an aluminum or copper foil may be used. Preferably, copper is used, especially an electrodeposited copper foil, produced by electrodepositing copper on a drum surface. With respect to such electrodeposited copper foil, the surface at which copper deposition is initiated (the surface at which formation of copper deposits brought into contact with the drum is initiated) is referred to as “shiny side”, and the surface at which copper deposition is completed is referred to as “matte side”. The surface condition of the shiny side is substantially the same as that of the drum. That is, the RMS surface roughness value (a conventional measurement of metal surface roughness for layers used in PCBs; see more below) of the drum is from about 0.1 to 0.5 microns with a maximum peak to valley roughness value from about 1.0 to 2.0 microns. (Maximum peak to valley roughness is another means of characterizing surface roughness of a metal layer such as copper foil used in PCBs) As a result, the “shiny” side of the electrodeposited copper formed on this drum (and against the drum's outer surface) has a similar roughness. On the other hand, with respect to the outer matte side of the formed copper layer, its surface roughness is greater than the surface roughness of the shiny side, typically having an RMS value of from about 1.0 to about 2.0 microns with a maximum peak to valley roughness in the range of about 3.0 to 10 microns.
There are various different methods of characterizing surface roughness in the industry including Ra (average roughness or the arithmetic average above and below the center line in a segment), Rq (or RMS, which is the square root of the average of the squared absolute distances of the surface profile from the mean line), Rt (maximum peak to valley or the height difference between the highest and lowest points in a segment) and Rz (the 10 point average surface roughness). RMS (Rq) values will be used herein and simply referred to as “RMS roughness” for ease of explanation purposes.
In the case of conventional electrodeposited copper foils, it is known to subject these foils to various treatments prior to inclusion thereof as part of a dielectric-conductive layer composite (or, more likely, a sub-composite if used in combination with other sub-composites to form a multi-layered built-up final board), including treating the foil for the purpose of increasing adhesion between the foil and dielectric layer(s) in the final structure. For example, mechanical polishing is a method of smoothing the surface of the copper foil with the use of mechanical means, usually in the form of a buffer. Unfortunately, if the foil is too thin, it may be damaged, e.g., severed or torn in sections, from the relatively high stresses exerted on the copper foil during this processing. Thus, mechanical polishing is considered suitable for preparing the surface of relatively thick copper foils only. In comparison, chemical and electrolytic polishing processes exert virtually no relatively high stresses on copper foils so it is believed that relatively thin foils may be successfully treated using one or both of these processes. However, such processes are typically expensive to operate, often requiring relatively expensive equipment, costly chemical baths, as well as prolonged periods during which the foil is so treated, thereby extending the total time of manufacture of the end product.
In U.S. Pat. No. 6,475,638 (Mitsuhashi, et al.), there is described a process for producing an electrodeposited copper foil with its surface prepared which includes the steps of subjecting the foil having a shiny side and a matte side to at least one mechanical polishing so that the average surface roughness (Rz) of the matte side becomes in the range of 1.5 to 3.0 microns. The matte side is then subjected to a selective chemical polishing so that the average surface roughness (Rz) of the matte side becomes in the range of 0.8 to 2.5 microns. The mechanical polishing followed by chemical polishing of the matte side enables the foil to exhibit excellent properties, according to the authors.
In U.S. Pat. No. 6,291,081 (Kurabe, et al.), there is described a process for producing an electrodeposited copper foil including the steps of subjecting an electrodeposited copper foil having a shiny side and a matte side to a first mechanical polishing and then subjecting the matte side having undergone the first mechanical polishing to a further mechanical polishing. A planar, highly polished face with excellent surface properties is allegedly obtained. Moreover, depressed parts are not polished, so that the amount of copper lost by the polishing steps is extremely minute.
In U.S. Pat. No. 5,897,761 (Tagusari, et al.), there is described an electrodeposited copper foil for use in the manufacture of printed wiring boards in which the original profile of the matte surface has been completely removed, preferably by buffing, leaving a surface having linear streaks and a certain roughness. The new surface is then given a nodule forming treatment which produces a second surface roughness, which may be followed by a corrosion resisting treatment. U.S. Pat. No. 5,858,517 (also Tagusari, et al.) also describes a similar process with what are considered minor modifications.
In U.S. Pat. No. 5,545,466 (Saida, et al.), there is described a copper-clad laminate characterized in that an electrolytic copper foil on the glossy (shiny) surface side of which a copper electrodeposit is formed, is bonded at its glossy surface side to one side or each of both sides of a substrate, which has a fine-pitch wiring (circuit) pattern and exhibits a high etching factor. This patent is a continuation-in-part of U.S. Pat. No. 5,437,914 (Saida, et al.), below.
In U.S. Pat. No. 5,482,784 (Ohara, et al.), there is described a printed circuit inner-layer copper foil having inverted tear drop-shaped fine nodules formed on both surfaces of the copper foil, the nodules each having a specific length and maximum diameter.
In U.S. Pat. No. 5,437,914 (Saida, et al.), there is described a copper-clad laminate characterized in that an electrolytic copper foil on the glossy surface side of which a copper electrodeposit is formed is bonded at its glossy surface side to one side or each of both sides of a substrate.
In U.S. Pat. No. 5,096,522 (Kawachi, et al.), there is described a process for producing a copper-clad laminate which includes the steps of contacting the surface of a conductive carrier with a catalyst liquid containing a noble metal selected from the group consisting of Pd, Pt, Ru, Au, and Ag, subsequently forming a copper foil layer on the treated surface by copper electroplating, laminating an insulating base on the copper foil layer by hot-press bonding, and then separating the conductive carrier from the resulting laminate. The copper foil layer in the resulting copper-clad laminate is claimed to have fewer pinholes and allegedly exhibits isotropic mechanical characteristics.
In four U.S. patents cited below with respect to the definition of what is meant by a “fluid treatment device”, there are described various embodiments of fluid treatment apparatus/assemblies which are specifically designed for applying precisely directly pressurized jets of fluid onto designated locations on the surface of a nearby material. As defined in these patents, such materials are typically passed through the apparatus/assembly with the fluid directed onto opposite sides thereof from the oppositely positioned sprayers, but may only be directed onto one of the sides, if desired. The various pressures attainable using these structures are defined in detail in many of these patents.
In Japanese Patent Unexamined Publication Hei 5-160208, there is disclosed a tape carrier having a lead pattern formed from an electrodeposited copper foil wherein the overall surface of the foil's matte side has been polished. This publication describes the use of an electrodeposited copper foil whose 1-2 micron matte side surface profile has been chemically polished. It is mentioned that a highly reliable carrier tape with desired lead strength can be provided by the use of the copper foil whose matte side overall surface has been so chemically polished.
According to the teachings of the present invention, there is defined a circuitized substrate in which a conductive layer (e.g., electroplated copper foil) is mated with another and bonded (e.g., laminated) to an interim dielectric layer. Each of the two foil surfaces which physically bond to the dielectric is smooth while the outer surfaces of both foils, albeit rougher than the facing sides, are also smooth. One of these resulting conductive layers may function as a ground or voltage plane while the other may function as a signal plane with a plurality of individual signal lines as part thereof. The signal lines may be extremely thin and also extremely narrow in width, in which case these are still able to enable the passage of high speed signals there-through. As stated, however, the invention is not limited to substrates with extremely thin and narrow signal lines, as it is clear from the teachings herein that substrates having thicker and wider lines than defined herein may be successfully produced.
It is believed that such a substrate and method of making same, as well as resulting electrical assemblies and information handling systems utilizing same, will represent significant advancements in the art.